Changes in Absolute Myocardial Blood Flow With Beta Blocker Therapy in Heart Failure: The addition of beta blockers to standard heart failure therapy improves left ventricular ejection fraction and mortality. However, the underlying mechanism for the beneficial effects of beta blockers in heart failure is not well understood. We compared changes in absolute myocardial blood flow before and after the addition of 6 months of metoprolol to standard heart failure therapy in 12 congestive heart failure patients with NYHA class II-IV; 6 ischemic and 6 non- ischemic. All patients underwent pre- and post-beta blocker N-13 ammonia PET and radionulide angiography studies. Mean left ventricular ejection fraction increased in the 12 patients from 16+8% pre- to 23+11% post-BB (p<0.0005). In regions with abnormal resting myocardial blood flow, beta blocker therapy improved perfusion in ischemic (.40+.18 pre- to .54+.18 post-therapy, p<0.01) but not in non-ischemic patients (.39+.15 pre- to .40+.21 post-therapy, p=NS). We concluded that beta blockers may increase myocardial blood flow in abnormal regions in ischemic patients but its mechanism of action in non- ischemic patients appears different. Perhaps altered substrate utilization such as fatty acid and/or glucose may provide additional insight into the underlying mechanism for the salutary effects of beta blockers in congestive heart failure. - coronary artery disease (CAD), single positron emission computerized tomography (SPECT) - Human Subjects